1. Field of the Invention
The present invention concerns electrocatalytic coatings for oxygen-evolving anodes.
2. Description of the Related Art
The anodic materials of the prior art for the electrometallurgy of copper, zinc and cobalt are essentially of two types: lead alloys, and cobalt-silicon alloys (cobalt only). Industrial lead anodes are made of lead alloys containing one or more elements selected in the following group: I B, IV A and V A. In particular, the lead-silver (0.2-0.8%) anode is commonly used especially in the zinc electrometallurgy, while for the cobalt electrometallurgy different alloys are used, such as lead-antimony (2-6%), lead-silver (0.2-0.8%), lead-tin (5-10%). These materials are characterized by:
high anodic potentials, above 1.9 V (NHE). PA1 lifetimes in the range of 1 to 3 years PA1 high electric resistivity and substantial electric disuniformity leading to the formation of thick solid layers of PbSO.sub.4 (intermediate passivating layer) and PbO.sub.2 (external electrocatalytic layer for oxygen evolution). PA1 faradic efficiency loss (below 90% for zinc, and below 95% for cobalt) PA1 uneven and dendritic aspect of the deposit PA1 contamination by lead of the produced metal. PA1 oxygen evolution: 2H.sub.2 O=4H+O.sub.2 +4e PA1 manganese dioxide deposition (parasitic reaction): 2Mn.sup.2+ +4H.sub.2 O=2MnO.sub.2 +8H.sup.+ +4e. PA1 a) addition of highly catalytic metals for oxygen evolution, for example ruthenium and cobalt, to the main components consisting of tantalum and iridium, to fix the voltage at low and controlled values. PA1 b) further addition of metals capable of stabilizing ruthenium and cobalt, such as titanium and tin.
These characteristics involve the following drawbacks:
The cobalt alloys, used for a part of the cobalt electrometallurgy, are substantially of three types characterized by the following compositions: cobalt-silicon (5-20%), cobalt-silicon (5-20%)-manganese (1.0-5.0%), cobalt-silicon (5-20%)-copper (0.5-2.5%).
The cobalt-silicon alloys, with respect to the lead alloys, are characterized by a longer lifetime but are affected by a higher electrical resistivity and brittleness, while the cobalt-silicon-copper alloys have a shorter lifetime and are all the same fragile.
As concerns cathode poisoning, this occurs only when copper alloys are used.
Table 1 summarizes some examples of general operating conditions of the prior art technology. Reference is made to the process for zinc and cobalt deposition.
TABLE 1 __________________________________________________________________________ Prior art operating materials Anode lifetime (years) Current Co--Si Process Electrolyte Density A/m.sup.2 Pb--Sn Pb--Ag Co--Si--Mn Co--Si--Cu __________________________________________________________________________ Zinc Zn.sup.2+ 300-500 g/l) // 2-4 // // H.sub.2 SO.sub.4 (150-200 g/l) Fluorides (50 ppm) Manganese (2-8 g/l) Zn.sup.2+ 300-500 g/l) 1-3 2-4 // // H.sub.2 SO.sub.4 (150-200 g/l) Fluorides (5 ppm) Manganese (2-8 g/l) Cobalt Co.sup.2+ 160-250 g/l) 4-5 4-5 3-4 2-3 H.sub.2 SO.sub.4 (pH 1-3) Manganese (10-30 g/l) pH = 4-5,5 __________________________________________________________________________
In the electrolysis of solutions containing, besides the salt of the metal to be deposited, also significant quantities of manganese (5-20 g/l and more), two reactions take place at the anode, and precisely:
This anodic by-product is an electrically resistive oxide (resistivity equal or higher than that of the PbO.sub.2 -PbSO.sub.4 mixture formed on lead anodes); as a consequence, its precipitation on the surface of the electrode, if compact and continuous with time, involves a progressive increase of the electrode potential, which negatively affects prior art electrodes. In industrial practice, to avoid or at least control this phenomenon, the anodes (lead alloys or cobalt alloys) are periodically cleaned by mechanical brushing carried out outside the electrolysis cell.
It is known that titanium electrodes, activated by conventional coatings based on tantalum and iridium oxides, when used in electrolytes containing manganese, are negatively affected by the same drawbacks as lead anodes, with the only difference that the mechanical cleaning is not applicable due to the insufficient mechanical stability of the coating. Therefore, possible alternatives to the mechanical removal of the MnO.sub.2 have been considered, such as periodical washing outside the cell with reducing solutions such as H.sub.2 O.sub.2 ; H.sub.2 O.sub.2 +nitrates or nitric acid; ferrous salts and nitrates, ferrous salts and sulphates, etc. or actions carried out in the cell, such periodical current reversal, periodical current interruption, scheduled shut-downs etc. As the results were either negative or unsuitable for industrial scale application, efforts have been focused on the spontaneous removal of manganese dioxide electrodeposited onto the anode directly in the electrolysis cell.